1. Field of the Invention
The present invention relates, in general, to hard disk drive (HDD) systems and magneto resistive data storage devices and systems, and, more particularly, to a write driver with a circuit, and associated method, for HDD applications capable of switching a current into a write head, such as a low impedance head, connected to the write driver through a transmission line or interconnect. The write driver circuit includes a boosting device or circuit to increase the output voltage launched into the transmission line while maintaining impedance matching between the write driver and the characteristic impedance of the transmission line.
2. Relevant Background
The demand for improved data storage techniques and systems continues to rapidly grow. Hard disk drives utilizing magneto resistive (MR) heads to read and write data onto one or more spinning magnetic platters or disks are one of the more important and wide spread devices in the data storage industry. Hard disk drives may be used in many applications, including enterprise computer systems, personal computers, set top boxes, audio, video, or television applications, and many other large and small computer devices. Many applications are still being developed, and the uses for hard disk drives are expected to increase.
Hard disk drives store binary encoded information as regions of magnetic flux on a media having a magnetic surface coating. It is desirable that these magnetic regions be encoded on the disk as densely as practical, so that a maximum amount of information may be stored. Disk and tape drive suppliers continue to increase areal densities, or the number of data bits per square inch, to meet the increasing demand for storage at competitive pricing. However, increasing areal density requires the write mechanism to produce smaller recorded patterns on the disk. Write head design and write driver design are key technologies needed to achieve these capacity increases.
The magnetic regions are created by passing current through a coil of a magnetic write head. Binary data can be encoded by switching the polarity of the current through a coil in the write head. The current in the write head coil is provided by a circuit in a write driver that is connected to the coil through a flexible transmission interconnect. The data rate (i.e., the rate at which bits can be written onto the media) is determined largely by the rate at which the current can be switched in the write head driver circuit. It is desirable to have a write driver circuit that quickly switches current to the desired polarity and magnitude to support high disk rotation speeds with small magnetic regions. Also, the driver circuit must raise the current amplitude to a level sufficient to ensure the flux generated by the write coil is adequate to saturate the magnetic media while limiting the current below levels that will result in “blooming” of the written magnetic region into adjacent regions of the media.
A conventional write driver circuit comprises an H-bridge configuration using four switches. In an H-bridge circuit, one leg of the bridge is always trying to drive current into the inductive load. In other words, the H-bridge is always coupling the power supply voltage onto one of the inductor nodes and ground to the other inductor node by appropriately activating the bridge switches. While there have been many improvements to conventional write driver circuits to enhance their performance, there are demands for improved performance. For example, many switching write driver circuits still are unable to achieve impedance matching to transmission interconnects and the lack of impedance matching results in pattern dependent distortion which limits the performance of the write circuitry. There are also continuing issues with the power required to operate or drive the write head with the write driver, and specifically, there is an increasing demand to launch output power into the transmission interconnect that is boosted relative to power levels supplied to the write driver.
FIG. 1 illustrates a theoretical ideal circuit 100 for use in a write driver that functions to produce a current pulse at the driver output node 120 or HWX node for the overshoot duration, OSDUR, with an amplitude, IOS/2. In the ideal circuit 100, the current pulse is generated by an ideal current source 114 that has an infinite output impedance that is independent of the value of the output voltage at node 120 (or output node HWX). As a result, the output dynamic range of the ideal circuit 100 is infinite. Of course, the ideal circuit 100 cannot be reproduced in manufactured write driver circuits.
A variety of circuit designs have been used in write heads to effectively provide the desired pulsed current at the write driver output. FIG. 2 illustrates one commonly implemented write driver circuit 200 for delivering a current pulse to a write driver output node 230. As shown, the current pulse is delivered to the output node 230 through the use of transistors 218, 220, e.g., metal-oxide semiconductor (MOS) transistors, configured as current mirror 216. A high reference or input voltage, VCC, is applied on nodes 210, 212 and a low reference voltage, VEE, on node 228, and a current source 224 is provided between node 228 and transistor 218 of the current mirror 216. In practice, the output impedance of transistor 220 (or transistor M1) is high as long as the transistor 220 works in the saturation region, and the output impedance becomes low when the transistor 220 enters the triode region of operation. The output dynamic range of the circuit 200 is equal to the supply voltage, VCC, less the overdrive voltage, VOVM1, of transistor 220. The amplitude of the current pulse delivered by the source 224 to a load connected to the output node 230 is IOS/2 for output voltages at the HWX or output node 230 within the output dynamic range. Unfortunately, the amplitude of the current pulse produced by the write circuit 200 at output node 230 decreases for output voltages at the output node 230 reaching or exceeding the output dynamic range.
Hence, a need exists for a circuit for driving write heads in a hard disk drive (HDD) system that more effectively provides a current pulse with a desired amplitude, such as IOS/2, for the duration of the overshoot, OSDUR. Preferably, such an improved circuit would control propagation of reflection from the write head or load by matching the impedance of the write driver circuit with the impedance of interconnects with the write head.